One of the grand open challenges in modern science is to identify cells or probe molecules and understand the mechanism and dynamics of biological processes at the molecular level with high sensitivity and spatiotemporal resolution, and particularly inside living cells and tissue or liquid. As a result of the wealth of information potentially accessible from such biological targets, there has been a growing demand for imaging tools for biomedical research and medicine. This research has led to the development of new techniques like magnetic resonance imaging (MRI), ultrasound, positron emission tomography (PET), and optical coherence tomography (OCT). However, these techniques require high costs and some fundamental technological barriers hinder their widespread use.
Optical imaging is a recent technique that utilizes photons as an information source with applications in a wide range of basic science and clinical studies like pharmacology, cellular biology, and diagnostics. For example, semiconductor nanocrystals, small organic dyes or fluorescent proteins are commonly used as optical labels in in vivo optical imaging. (See, e.g., X. Michalet et al., Science 307, 538 (Jan. 28, 2005); B. Dubertret et al., Science 298, 1759 (Nov. 29, 2002); M. K. So, C. Xu, A. M. Loening, S. S. Gambhir, J. Rao, Nat Biotechnol 24, 339 (March, 2006); N. C. Shaner, P. A. Steinbach, R. Y. Tsien, Nat Methods 2, 905 (December, 2005); and B. N. Giepmans, S. R. Adams, M. H. Ellisman, R. Y. Tsien, Science 312, 217 (Apr. 14, 2006), the disclosures of which are incorporated herein by reference.) Indeed, recent advances in fluorescence microscopy alone have profoundly changed how cell and molecular biology is studied in almost every aspect. (For example, see, Lichtman, J. W. & Conchello, J. A. Nat. Methods 2, 910-919 (2005); Michalet, X. et al. Annu. Rev. Biophys. Biomolec. Struct. 32, 161-182 (2003); Jares-Erijman, E. A. & Jovin, T. M. Nat. Biotechnol. 21, 1387-1395 (2003); Bastiaens, P. I. H. & Squire, A., Trends Cell Biol. 9, 48-52 (1999); and Suhling, K., et al, Photochem. Photobiol. Sci. 4, 13-22 (2005), the disclosures of which are incorporated herein by reference.)
However, the ultimate need of characterizing biological targets is largely unmet due to fundamental deficiencies associated with the use of fluorescent agents. For example, fluorescent probes face two major limitations that have a significant impact on the signal strength: 1) dye saturation, because the number of photons emitted by the fluorophore in a given time is restricted by the excited state lifetime, and 2) dye bleaching, which limits the total number of photons per dye. In addition, autofluorescence from tissue organic components due to illumination absorption can severely limit the signal-to-noise ratio. Finally, fluorescence is fundamentally an optically incoherent process, and as a result extracting 3D information from the source is inherently difficult.
Accordingly, a need exists for a new probe for imaging/detecting biological structures and processes that avoids the inherent technological limitations found in the fluorescent imaging techniques of the prior art.